"Interestingly, no one really talks about the other side of this situation: global warming acceleration. The mid-1970s through to the mid-1990s was a period of positive Pacific Decadal Oscillation [PDO] and saw an acceleration in warming. If you consider the arguments about the effect of the negative phase on warming, then a positive PDO should result in the opposite. That is, reduce the relative rate of deeper ocean heat increases and instead increase the rate at which surface warming is observed."

It is indeed very "Interesting, no one [other than climate skeptics] really talks about the other side of this situation" that could naturally account for all observed global warming since the ice age scare of the 1970's.

Where is the heat? That is the question on the minds of many scientists, and many climate change sceptics. The 'global warming hiatus' — the fact that globally averaged air temperatures have not increased as quickly in the past decade as they have in previous decades1, 2 — is a hot topic, so to speak. It even has its own spotlight in Chapter 9 of the Working Group I report of the IPCC 5th Assessment Report3.

Known modes of variability in the climate system do influence the exchange of heat between ocean and atmosphere and the distribution of heat within the ocean.

The Pacific Decadal Oscillation (PDO)11 shares many similarities with ENSO (Fig. 2). The negative phase exhibits cooler than normal sea surface temperatures in the equatorial Pacific and warmer than normal temperature in the mid-latitude Pacific. Many researchers believe that ENSO's impact on the large-scale atmospheric circulation drives the PDO12, although other oceanic mechanisms contribute13. One of the obvious distinctions between ENSO and PDO is the timescale; PDO phases last 10–40 years, though with considerable year-to-year noise. Another distinction is that the magnitude of temperature anomalies associated with PDO are greater in the mid-latitudes than near the equator, which is the opposite of that for ENSO. Although the equatorial temperature signature is weaker for PDO, it has a greater meridional extent. The associated wind field shows a broad swath of increased trade winds and a speed-up of the subtropical cells — the Pacific shallow overturning circulation that connects the subtropical to the equatorial region. This is thought to play an important role in the PDO based on observations14 and modelling results15. Again, this can increase heat storage in the central and western Pacific, perhaps to greater depths (that is, below 300 m) than resulting from La Niña events.

The Atlantic Multi-decadal Oscillation (AMO) is indexed by the average sea surface temperature anomalies over the North Atlantic Ocean. Its timescale is longer than that of the PDO, and seemingly less noisy. Variations in the strength of the Atlantic meridional overturning circulation, part of the global thermohaline circulation, are thought to be the main driver of the AMO16. A weakened overturning circulation means less formation of cold deep water, which would result in a relative warming through to the depths of the ocean.

A weakened overturning circulation would also draw less warm water polewards, whereas the North Atlantic has been warm since the mid-1990s. This is not conclusive proof that overturning circulation has not changed as there is considerable difficulty in separating the forced and natural variability over the North Atlantic17. It could be explained by an increased spin of the ocean gyre through increased trade winds, which would transport warm tropical waters to higher latitudes regardless of changes in sub-polar Atlantic Ocean deep convection.

It is worth noting that the Southern Ocean, the other key source of deep water, has shown a reduction in Antarctic bottom water formation since the 1980s. Observations suggest that this reduction contributed approximately 10% of the ocean heat uptake during that time18.

Natural variability seems to be capable of accounting for changes in ocean heat uptake of the magnitude experienced. Many recent studies point to the role of PDO in this recent hiatus. What is particularly compelling is that this period has also been one of negative PDO. Further suggestive evidence is that the last period with decade-scale trends in global mean temperature as weak as that experienced since the turn of the century occurred through the 1950s and early 1960s, which was another period dominated by very negative PDO conditions. This shows that hiatus periods are unusual but not unprecedented.

Interestingly, no one really talks about the other side of this situation: global warming acceleration. The mid-1970s through to the mid-1990s was a period of positive PDO and saw an acceleration in warming. If you consider the arguments about the effect of the negative phase on warming, then a positive PDO should result in the opposite. That is, reduce the relative rate of deeper ocean heat increases and instead increase the rate at which surface warming is observed.

Another neglected topic is that negative PDO-like conditions are not inconsistent with climate change. One theory of the tropical Pacific response to increased radiative heating [which would have to be solar, since IR cannot penetrate beyond a few microns] is that the western Pacific warms at a faster rate than the eastern Pacific due to the upwelling in the east. This gradient in heating strengthens the existing temperature gradient along the equatorial Pacific, which strengthens the trade winds, potentially leading to a more La Niña-like mean state19. There is also the question of possible connections between the Pacific and Atlantic, for which there is little definitive evidence at this point.

Unfortunately, this is not a game. How our climate system responds to human activities has serious implications for society, as does the role of natural variability in our realization of climate change. Although the international community has invested in numerous observational systems — in the oceans, on land and in space — we still lack the long-term and continuous observations, and their synthesis, that are critical to understanding the climate system as a whole. Observing systems must be sustained, and where critical gaps are identified — such as the deep oceans — they should be enhanced. For example, the TOGA/TAO array of buoys in the tropical Pacific, which has been critical to our understanding, monitoring and prediction of ENSO, has fallen into decay and is threatened with extinction. Those buoys have done more than any other single project to mitigate the impacts of climate on society worldwide. Not only should that array be refreshed, it should be expanded into the mid-latitudes of the Pacific Ocean. That could provide an unprecedented view of the processes behind things like the PDO, and perhaps lead to predictions of the next hiatus or acceleration.

The information needed to help manage the risks and opportunities of future climate changes, whether natural or man-made, must be based on solid science. Science starts with good observations and their synthesis, but it cannot stop there. It must serve improved understanding, monitoring, and prediction of interannual to decadal variability and its manifestation against a changing mean climate.